There are many challenges related to the fabrication and assembly of optical systems configured in trenches formed in a substrate, such as a silicon optical bench (SiOB) formed within a silicon substrate. In a silicon optical bench, a silicon substrate is used as the base for the optical system. In a free space optical waveguide SiOB, light travels in free space of trenches that are formed on the surface of the silicon wafer. Various etches are used to form v-grooves and trenches of an SiOB. For example, KOH is used to form v-grooves in the silicon surface and deep reactive ion etching (DRIE) is used to form trenches in the silicon through which optical beams propagate. Optical components such as optical fibers, lenses, transmission windows, mirrors, beam splitters, polarizers and other components that direct (i.e., guide, reflect, split, or polarize) the optical beams through a trench system are positioned within the v-grooves and/or trenches.
The trenches that are formed in the trench system to hold optical components, such as the windows, mirrors, beam splitters or polarizers, are typically slightly oversized to enable the insertion of the optical component. The optical components are generally cut or sawn from a large substrate, such as a silicon wafer, into the size(s) needed for the SiOB. The optical components formed by sawing up the components have a square or rectangular shape.
Trenches with non-vertical side wall profiles are created by the silicon etches typically used to form the trenches, such as DRIE. In some cases, the trench has a profile that widens or slopes outward from the bottom of the trench to the top surface of the silicon so the trench is wider at the surface of the substrate than at the depth of the trench. In other cases, a re-entrant profile trench is generated that is narrow at the top surface and widens going from the top surface to the bottom of the trench.
Over-sized trenches and/or non-vertical trench side walls tilt the components placed in the trenches with respect to the optical path. When components such as mirrors, transmission windows, and beam splitters are tilted, they direct optical energy out of the desired optical path, thereby diminishing the optical signal at the output end, and possibly directing unwanted optical energy into other optical systems.